The present invention relates to a continuous in-line process for economically and efficiently producing aluminum alloy sheet and, particularly, to a continuous, in-line process for producing aluminum alloy can stock.
Conventional manufacturing of flat rolled finish gauge stock has used batch processes which include an extensive sequence of separate steps. In the typical case, a large ingot is cast for rolling, and is then slowly cooled to ambient temperature. The ingot is then stored for inventory management. When an ingot is needed for further processing, it is first treated to remove defects such as segregation, pits, folds, liquation and handling damage by machining its surfaces. This operation is called scalping. Once the ingot has surface defects removed, it is preheated at a required temperature for several hours to ensure that the components of the alloy are uniformly distributed through the metallurgical structure, and then cooled to a lower temperature for hot rolling. While it is still hot, the ingot is subjected to breakdown hot rolling in a number of passes using reversing or non-reversing mill stands which serve to reduce the thickness of the ingot. After breakdown hot rolling, the ingot is then typically supplied to a tandem mill for hot finishing rolling, after which the sheet stock is coiled, air cooled and stored. The coil is then typically annealed in a batch step. The coiled stock is then further reduced to final gauge by cold rolling using unwinders, rewinders and single and/or tandem rolling mills.
Batch processes typically used in the aluminum industry require about seventeen different material handling operations to move ingots and coils between what are typically fourteen separate processing steps. Such operations are labor intensive, consume energy, and frequently result in product damage, re-working of the aluminum and even wholesale scrapping of product. And, of course, maintaining ingots and coils in inventory also adds to the manufacturing cost.
Aluminum scrap is generated in most of the foregoing steps, in the form of scalping chips, end crops, edge trim, scrapped ingots and scrapped coils. Aggregate losses through such batch processes typically range from 25 to 40%. Reprocessing the scrap thus generated adds 25 to 40% to the labor and energy consumption costs of the overall manufacturing process.
It has been proposed, as described in U.S. Pat. Nos. 4,260,419 and 4,282,044, to produce aluminum alloy can stock by a process which uses direct chill casting or minimill continuous strip casting. In the process there described, consumer aluminum can scrap is remelted and treated to adjust its composition. In one method, molten metal is direct chill cast followed by scalping to eliminate surface defects from the ingot. The ingot is then preheated, subjected to hot breakdown followed by continuous hot rolling, batch anneal and cold rolling to form the sheet stock. In another method, the casting is performed by continuous strip casting followed by hot rolling, coiling and cooling. Thereafter, the casting is annealed and cold rolled. The minimill process as described above requires about ten material handling operations to move ingots and coils between about nine process steps. Like other conventional processes described earlier, such operations are labor intensive, consume energy and frequently result in product damage. Scrap is generated in the rolling operations resulting in typical losses throughout the process of about 10 to 15%.
In the minimill process, annealing is typically carried out in a batch fashion with the aluminum in coil form. Indeed, the universal practice in producing aluminum alloy flat rolled products has been to employ slow air cooling of coils after hot rolling. Sometimes the hot rolling temperature is high enough to allow recrystallization of the hot coils before the aluminum cools down. Often, however, a furnace coil batch anneal must be used to effect recrystallization before cold rolling. Batch coil annealing as typically employed in the prior art requires several hours of uniform heating and soaking to achieve the anneal temperature. Alternatively, after breakdown cold rolling, prior art processes frequently employ an intermediate annealing operation prior to finish cold rolling. During slow cooling of the coils following annealing, some alloying elements present in the aluminum which had been in solid solution precipitate, resulting in reduced strength attributable to diminished solid solution hardening.
The foregoing patents (U.S. Pat. Nos. 4,260,419; and 4,282,044) employ batch coil annealing, but suggest the concept of flash annealing in a separate processing line. These patents suggest that it is advantageous to slow cool the alloy after hot rolling and then reheat it as part of a flash annealing process. That flash anneal operation has been criticized in U.S. Pat. No. 4,318,755 as not economical.
There is thus a need to provide a continuous, in-line process for producing aluminum alloy sheet which avoids the unfavorable economics embodied in conventional processes of the type described.
Substantial improvements in the manufacture of aluminum alloy sheet and can stock have been realized by the so-called micromill process described in the aforementioned co-pending applications. In the process disclosed and claimed therein, it is possible to produce aluminum alloy sheet stock and can stock in an economical manner by first providing an aluminum alloy feedstock, preferably by continuous strip casting. The feedstock, already hot from the casting operation, is hot rolled to reduce its thickness and is then annealed and solution heat treated without intermediate cooling to maintain the alloys in the aluminum in solid solution without precipitation. There-after, the feedstock is rapidly quenched and, where desired, subjected to cold rolling.
One of the principal advantages of the techniques described in the aforementioned application is that the steps of the micromill process are carried out continuously in an in-line sequence of steps which either eliminates or substantially minimizes the material handling operations which contribute undesirably to the cost of prior art processes. One of the principal advantages of the method described in those foregoing applications is that they can be located immediately adjacent to a can making plant to streamline material handling operations, lower shipping costs and minimize returning scrap costs.
A variation of that process is also disclosed and claimed in U.S. Pat. No. 5,356,495 in which use is made of two sequences of continuous, in-line operations. In the first sequence, the aluminum alloy feedstock is first subjected to hot rolling, coiling and coil self annealing and the second sequence includes the continuous, in-line sequence of uncoiling, quenching without intermediate cooling, cold rolling and coiling. The process as described in the latter patent has the advantage of eliminating the capital costs of an annealing furnace while nonetheless providing aluminum sheet and can stock having strength associated with aluminum alloys which have been heat treated.
It has now been discovered that aluminum alloys and can stock can be produced by utilizing two different sequences of inline continuous operation in which the first sequence includes a quenching step and the second sequence includes a rapid annealing step to provide aluminum alloy sheet stock and can stock having highly desirable metallurgical properties. It has been found that the rapid quenching in the first sequence of steps and the rapid heating followed by quenching in the second sequence of steps do not permit substantial precipitation of alloying elements present in the alloy and, thus, affords an aluminum alloy sheet and can stock having highly desirable metallurgical properties.
It is accordingly an object of the present invention to provide a process for producing aluminum alloy sheet stock which can be carried out in two in-line sequences without the need to employ many separate batch operations.
It is a more specific object of the invention to provide a process for commercially producing an aluminum alloy finish gauge sheet stock and aluminum alloy can stock in a semi-continuous process which can be operated economically and provide a product having equivalent or better metallurgical properties.
These and other objects and advantages of the invention appear more fully hereinafter from a detailed description of the invention.
The concepts of the present invention reside in the discovery that it is possible to combine casting, hot rolling and rapid quenching in a first continuous sequence of steps whereby the rapid quenching does not permit substantial precipitation of alloying elements from solid solution, thereby ensuring that the alloying elements remain in solid solution. Thereafter, in a second sequence of continuous, in-line steps, the aluminum alloy sheet can be flash annealed and rapidly quenched to ensure that alloying elements are in solid solution. The annealing followed by quenching in the second sequence of steps maximizes alloying elements in solid solution to strengthen the final product.
As used herein, the term xe2x80x9cannealxe2x80x9d or xe2x80x9cflash annealxe2x80x9d refers to a heating process to effect recrystallization of the grains of aluminum alloy to produce uniform formability and to control earing. Flash annealing, as referred to herein, refers to a rapid annealing process which serves to recrystallize the aluminum grains without causing substantial precipitation of intermetallic compounds. Slow heating and cooling of the aluminum alloy are known to cause substantial precipitation of intermetallic compounds. Therefore, it is an important concept of the invention that the heating, flash annealing and quenching be carried out rapidly. The continuous operation in place of batch processing facilitates precise control of process conditions and therefore metallurgical properties. Moreover, carrying out the process steps continuously and in-line eliminates costly materials handling steps, in-process inventory and losses associated with starting and stopping the processes.
The process of the present invention thus involves a new method for the manufacture of aluminum alloy sheet and can body stock utilizing the following process steps in two continuous, in-line sequences. In the first sequence, the following steps are carried out continuously and in-line.
(a) A hot aluminum feedstock is hot rolled to reduce its thickness;
(b) The hot reduced feedstock is thereafter rapidly quenched without substantial precipitation of alloying elements such as manganese to a temperature suitable for cold rolling;
(c) The quenched feedstock is, in the preferred embodiment of the invention, subjected to cold rolling to produce intermediate gauge sheet ; and
(d) The feedstock is coiled for further processing.
Thereafter, in a second sequence, the following steps may be carried out continuously and in-line:
(a) The feedstock is uncoiled and, optionally, can be subjected to cold rolling if desired to further reduce the thickness of the stock;
(b) The feedstock is subjected to a flash anneal to effect recrystallization of the aluminum grains at a sufficiently rapid rate to avoid substantial precipitation of alloying elements as intermetallic compounds and, thereafter, the feedstock is subjected to a rapid quench, also effected rapidly so as to substantially avoid precipitation of alloying elements as intermetallic compounds; and
(c) The quenched feedstock is thereafter subjected to further cold rolling and coiling to finish gauge.
It is an important concept of the invention that the flash anneal and the quench operation be carried out rapidly to ensure that alloying elements, and particularly manganese, as well as compounds of copper, silicon, mangnesium and aluminum, remain in solid solution. As is well known to those skilled in the art, the precipitation hardening of aluminum is a diffusion controlled phenomena which is time dependent. It is therefore important that the flash annealing and quenching operations of the second sequence of steps be carried out sufficiently rapidly that there is insufficient time to result in substantial precipitation of intermetallic compounds of copper, silicon, magnesium, iron, aluminum and manganese. At the same time, the annealing and quenching operations of the second step likewise minimize earing. That is particularly important when the aluminum alloy is a can stock alloy since earing is a phenomenon frequently found in the formation of cans from can body stock in which the plastic deformation to which the aluminum alloy is subjected is non-uniform. Thus, minimizing precipitation of intermetallic compounds raises the strength, allows recrystallization to be done at a lighter gauge, minimizes finish cold work and thereby reduces earing.
In accordance with a preferred embodiment of the invention, the strip is fabricated by strip casting to produce a cast thickness less than 1.0 inches, and preferably within the range of 0.06 to 0.2 inches. In another preferred embodiment, the width of the strip, slab or plate is narrow, contrary to conventional wisdom. This facilitates ease of in-line threading and processing, minimizes investment in equipment and minimizes cost in the conversion of molten metal to the sheet stock.
In accordance with yet another preferred embodiment of the invention, the feedstock is strip cast using the concepts described in co-pending application U.S. Pat. Nos. 5,515,908 and 5,564,491, the disclosures of which are incorporated herein by reference. In the method and apparatus described in the foregoing pending applications, the feedstock is strip cast on at least one endless belt formed of a heat conductive material to which heat is transferred during the molding process, after which the belt is cooled when it is not in contact with the metal. It is believed that the method and apparatus there described represents a dramatic improvement in the economics of strip casting.
In addition, the use of strip casting as described in the foregoing applications provides another significant advantage in that the molten metal is solidified in the strip casting apparatus, followed immediately by rolling and by quenching. That sequence of operations likewise ensures that no substantial precipitation of alloying elements in the form of intermetallic compounds can occur. The strip caster is operated at high speeds and, because it is followed by substantially immediate quenching, there is insufficient time within which to permit precipitation of alloying elements. That serves to provide improved metallurgical properties in the aluminum strip thus formed.